Complex Edge Stiffeners for Thin-Walled Members

Abstract

The objective of this paper is to investigate the behavior, and provide recommendations on, the design of open cross-section thin-walled cold-formed steel members that employ complex stiffeners. Complex stiffeners are formed when the free edge of an open section is folded multiple times, as opposed to simple stiffeners which employ a single fold (a lip). Simple stiffeners become ineffective when long lips are required to stabilize the flange, as the lip itself initiates the instability. In the elastic buckling regime, complex stiffeners are shown to hold a distinct advantage in local buckling over simple stiffeners. Closed-formed solutions for both local and distortional buckling of members with complex stiffeners can provide conservative and reliable solutions, but remain cumbersome and somewhat limited in applicability, therefore numerical solutions are preferred. Nonlinear finite element analysis is used to examine the post-buckling and ultimate strength regime. Complex stiffeners are shown to provide improved ultimate strength performance over simple stiffeners, but with a slight increase in imperfection sensitivity. Further, for an equivalent amount of material, complex stiffeners only provide advantages for specific stiffener lengths, though global optimal designs (maximum strength while minimizing material) still favor complex stiffeners over simple stiffeners in the investigated examples. The cold-formed steel design specification in current use is shown to be a poor predictor for the ultimate strength of bending members with complex stiffeners. However, the direct strength method, recently adopted as an alternative design method in the North American Specification for cold-formed steel members is shown to be a reliable predictor of ultimate strength. The direct strength method is recommended for design and optimization of members with complex stiffeners.